The invention relates to a Bragg grating device for measuring an acceleration.
The paper by T. A. Berkoff, A. D. Kersey: xe2x80x9cExperimental Demonstration of a Fiber Bragg Grating Accelerometerxe2x80x9d, IEEE Photonics Technology Letters, Vol. 8, No. 12, December 1996, pages 1677-1679, discloses a Bragg grating device for measuring an acceleration which has:
an optical Bragg grating, formed in elastic material, for supplying light, and
a deflectable mass connected to the grating for generating an inertial force that is dependent on the acceleration which acts on the mass, in order to produce elastic extension of the grating.
The connection between the mass and the grating is produced by a resilient layer, which is supported by a fixed baseplate and in which the grating is embedded. The extension of the grating is produced by the mass moving, in particular vibrating, at right angles to the direction of this extension.
The known device is operated as follows: light from a broadband source is supplied to the grating. The grating reflects a proportion of the light supplied at a grating-specific Bragg wavelength, which changes with the extension of the grating. The extension of the grating is produced by the inertial force generated by the mass, which is proportional to the acceleration that acts on the device and is to be measured. The reflected proportion of the light is supplied to an evaluation device, which determines which Bragg wavelength is contained in this proportion.
The evaluation device has a Mach-Zehnder interferometer having two arms of mutually different optical length. The reflected proportion of the light is coupled into the two arms, superimposed after passing through the two arms and is brought to interference and then fed to a detector. The determination of the Bragg wavelength contained in the reflected proportion is carried out with the aid of a phase modulator arranged in one of the two arms for the phase modulation of the part of the reflected proportion of the light that is guided in this arm relative to the part of this proportion which is guided in the other arm.
In the publication by J. R. Dunphy: xe2x80x9cFeasibility study concerning optical fiber sensor vibration monitoring subsystemxe2x80x9d in SPIE Vol. 2721, pp. 483-492, a Bragg grating device for vibration monitoring is described, which has at least one optical Bragg grating formed in elastic material for supplying light. In this case, evaluation devices for measuring the grating-specific Bragg wavelength of the grating are considered and compared with each other, said devices having a spectrometer, an interference filter, a tunable fiber grating, a scanning Fabry-Perot filter, a wavelength-dispersive element with a sampled detector or a tuned acousto-optical filter.
For a Bragg grating device having four Bragg gratings, among these evaluation devices, one device is viewed as relatively advantageous which, for each grating, has a tuned acousto-optical filter for evaluating the proportion of the light supplied and reflected by this grating with regard to the grating-specific Bragg wavelength contained in this proportion.
The publication by L. Zhang et al: xe2x80x9cSpatial and Wavelength Multiplexing Architectures for Extreme Strain Monitoring System using Identical-Chirped-Grating-Interrogation Techniquexe2x80x9d, in 12th International Conference on Optical Fiber Sensors, Oct. 28-31, 1997, pp. 452-455, reveals that the reflectivity R of a Bragg grating is a function both of the wavelength and also of the stress on the grating.
In this case, in a limited wavelength range around a grating-specific central Bragg wavelength, the reflectivity is greater than zero and essentially equal to zero outside this range. As the stress changes, the entire wavelength range is displaced linearly.
When xe2x80x9cchirpedxe2x80x9d Bragg gratings are used, that is to say gratings with a varying grating constant, a reflectivity R of the grating can be obtained which depends on the wavelength approximately in the form of a rectangular curve, so that the reflectivity within the wavelength range is substantially constant.
The object of the invention is to provide a Bragg grating device for measuring on acceleration which permits particularly simple evaluation.
To solve this problem, the Bragg grating device for measuring an acceleration according to the invention has at least two optical Bragg gratings, each formed of elastic material, for supplying optical radiation, and at least one deflectable mass connected to both gratings for generating an inertial force that is dependent on the acceleration which acts upon the device, so as to produce elastic extension of one of the two gratings and simultaneous elastic contraction of the other grating.
The solution achieved by the present invention applies, inter alia, to the case that often occurs in which on one occasion, the inertial force that is exerted produces a state of deformation of the two gratings in which one of the two gratings is extended elastically while, at the same time, the other grating is contracted elastically and, on another occasion, a state of deformation of the two gratings which is different from this state of deformation is produced, in which, conversely, the other grating is extended elastically and, at the same time, the one grating is contracted elastically. In particular in the case of accelerations in the form of vibrations, the case therefore occurs in which the one and the other states of deformation of the two gratings alternate.
As used herein, elastic material means a material in which the inertial force generated can produce such a large elastic deformation that a change produced by this deformation in a grating constant of the Bragg grating produces a measurable change in the grating-specific Bragg wavelength of a grating.
Deflectable mass as used herein means a mass which can be moved relative to the two gratings that are accelerated by the acceleration to be measured, for example relative to an accelerated frame to which the two gratings are fixed.
The inertial force produced by the mass can act on the two gratings directly or indirectly, for example via a force transmission device.
The two Bragg gratings are preferably arranged one behind another in a propagation direction of the optical radiation supplied. For example, these two gratings can be formed one behind another in the propagation direction in an optical conductor made of elastic material for guiding the radiation. The conductor can be, in particular, an optical wave guide, for example an optical glass fiber used in conventional Bragg grating sensors.
In a preferred and advantageous refinement of the device according to the invention, the mass is arranged between the two gratings and connected directly to each of the two gratings so that the inertial force produced by the mass acts directly on the gratings.
The mass can also be connected indirectly to the gratings via a force transmission device in order to convert the inertial force produced by the mass into a force that acts on each grating in such a way that one of the two gratings is extended and, at the same time, the other grating is contracted. In this case, the inertial force acting indirectly on the gratings and/or the extension or contraction of the gratings can be enlarged or reduced.
It is expedient if the force transmission device generates a force which is oriented in the propagation direction and/or the opposite direction and which acts directly on one grating in an extending manner and on the other grating in a contracting manner. For example, in this case the force transmission device can have a lever which can rotate about an axis of rotation that is substantially fixed relative to the device, or another mechanism that acts appropriately.
The two gratings can be formed or prepared in such a way that, in the state in which they are free of inertial force, that is to say in the acceleration-free state of the device, they have the same central Bragg wavelengths. With regard to the possibility of utilizing an advantageous linear characteristic curve range brought about by the two gratings, it is advantageous, however, if the two gratings have mutually different central Bragg wavelengths in the inertial-force-free state, the central Bragg wavelength of one of the two gratings lying within a grating bandwidth of the other grating.
In a refinement of the device according to the invention, at least one of the two Bragg gratings has a fixed grating constant. A particular refinement is in this case distinguished by a fixed grating constant of each of the two gratings. An improved linear characteristic curve range can be obtained with a refinement in which at least one of the two Bragg gratings has a variable grating constant (chirped grating), it being advantageous and expedient if both gratings each have a variable grating constant.
One advantage of the device according to the invention is to be seen in the fact that, during the evaluation of the proportion of the supplied optical radiation, coming from the two gratings, with respect to their grating-specific Bragg wavelengths, it is possible to use a simple broadband optical detector instead of a complicated narrow-band detector. Accordingly, an advantageous refinement of this device has a broadband optical detector for receiving a proportion, coming from both gratings, of the supplied optical radiation.
An advantageous method of operating a device according to the invention has in general terms the following steps:
supplying an optical radiation of a wavelength range containing the central Bragg wavelength of each of the two Bragg gratings to the gratings, and
measuring the intensity of a proportion of the supplied radiation coming from the gratings, as a measure of the acceleration to be measured.
An advantageous application of a device according to the invention consists in measuring a mechanical vibration frequency, the vibration frequency being lower than a resonant frequency of the device.
Another advantageous application of a device according to the invention consists in measuring a vibration amplitude of a mechanical vibration frequency which is higher than a resonant frequency of the device.